1. Field of the Invention
The present invention relates to a rectifier circuit that enables to generate a direct-current voltage from high-frequency low-intensity radio waves and a radio communication device that includes the rectifier circuit.
2. Description of the Related Art
Radio Frequency Identification (RFID) has been attracting attention as a technology for identification and management of human beings or objects. An RFID tag is cited as a representative example of the RFID technology, i.e., a non-contact authentication technology. A typical RFID tag includes a tiny radio-frequency integrated circuits (IC) chip and an antenna. The RFID tag receives high-frequency radio waves transmitted from a base station, so-called “a reader/writer”, via the antenna, and generates a direct-current voltage from an alternating current that is induced from the high-frequency radio waves at the antenna. Specifically, the direct-current voltage is generated by a rectifier circuit included in the RFID tag, and the generated voltage is used not only as a power supply voltage of the RFID tag but also as a communication signal.
The rectifier circuit is generally composed of a diode-connected metal-oxide-semiconductor (MOS) transistor. In a typical diode-connected MOS transistor, a gate and a drain of the MOS transistor are directly connected to each other. The rectifier circuit rectifies an effective value of an alternating-current signal exceeding a threshold voltage of the MOS transistor to a direct-current signal. In other words, based on the alternating-current signal below the threshold voltage, the rectifier circuit cannot generate a direct-current signal. On the other hand, even when the effective value of the alternating-current signal exceeds the threshold voltage, if a difference between the effective value and the threshold voltage is small, the rectification efficiency becomes low. This is because a rectifying object of the rectifier circuit is restricted to the alternating-current component obtained by subtracting the threshold voltage from the alternating-current signal.
To solve the problem, JP-A 2006-34085 (KOKAI) discloses a high-sensitive rectifier circuit. In a MOS transistor of the disclosed high-sensitive rectifier circuit, a drain and a gate are connected to each other via a capacitor. The capacitor holds a voltage approximately equivalent to a threshold voltage of the MOS transistor. Therefore, even if an effective value of an alternating-current signal is below the threshold voltage, the high-sensitive rectifier circuit can rectify the alternating-current signal to a direct-current signal.
However, if a charge leak occurs in the MOS transistor for voltage supply, voltages at both ends of the capacitor between the drain and the gate may gradually decrease. In other words, the rectification efficiency may decrease with the passage of time. Even if voltages are transferred from a plurality of capacitors to the capacitor between the drain and the gate by a switching circuit, voltages at both ends of each capacitor may also decrease. To solve the problem, a refresh operation is performed on the capacitor between the drain and the gate. Namely, a voltage is applied to the capacitor at regular intervals. The refresh operation can be performed by using a bias-voltage generating circuit and a pulse generating circuit. Therefore, an external power source such as a battery is required for running the bias-voltage generating circuit and the pulse generating circuit constantly.
As a result, a production cost and a size of the device increase because the external power source is built in the rectifier circuit. Moreover, an enough capacity design is required in consideration of continuous operating time of the circuits.